Alkali metal chlorate, and especially sodium chlorate, is an important chemical in the cellulose industry, where it is used as raw material in the production of chlorine oxide, which is an important bleaching chemical for cellulose fibres.
Alkali metal chlorate is produced by electrolysis of a chloride-enriched aqueous electrolyte according to the overall formula: EQU MeCl+3H.sub.2 O.fwdarw.MeClO.sub.3 +3H.sub.2 ( 1)
wherein Me =alkali metal.
The process is a cyclic process; where, in a first step, the chloride electrolyte is passed to an electrolyzer for the formation of hypochlorite, whereupon the resulting chlorate electrolyte is conducted to reactor vessels for further reaction to form chlorate. The chlorate formed is separated by crystallization while the mother liquor is recycled for preparing chloride electrolyte for further electrolysis to form hypochlorite.
In the cyclic chlorate process, pH is adjusted in several positions within the range 5.5-12, to optimize the process conditions for the respective unit operation. Thus, a weakly acid or neutral pH is used in the electrolyzer and in the reaction vessels to promote the reaction from chlorine via hypochlorite to chlorate, while the pH in the crystallizer is alkaline to prevent gaseous hypochlorite and chlorine from being released and to reduce the risk of corrosion.
In acidification, hydrochloric acid is normally used, but chlorine also occurs. To make the solutions alkaline, use is most often made of alkali metal hydroxide. Hydrochloric acid and alkali metal hydroxide are added in the form of aqueous solutions. Commercially available technical-grade solutions of hydrochloric acid and alkali metal hydroxide contain impurities which may derive from the chlorine/alkali plant and/or from subsequent transportation and storage. A chloride electrolyte to be electrolyzed in a chlorate cell must not contain high contents of impurities. Thus, Ca.sup.2+, Mg.sup.2+ and SO.sub.4.sup.2- give rise to deposits on the cathodes and, hence, higher operating voltage and energy costs, while heavy metals decompose the formed hypochlorite into chloride and oxygen, and not into chlorate, as desirable.
Alkali metal hydroxide is used for alkalization of the chlorate electrolyte before crystallization of chlorate, in alkaline purifying processes and in the regeneration of ion-exchange resins. Further, alkali metal hydroxide is used for eliminating the presence of gaseous chlorine compounds. Chlorine compounds give rise to problems of odor, health and corrosion, and also contaminate the hydrogen gas which is formed and which is often used as raw material for different syntheses. Hydrochloric acid is used for acidification of the chlorate electrolyte before electrolysis. In this case, mixing must be very thorough to prevent the formation of, inter alia, chlorine and chlorine dioxide in local, strongly acid regions, entailing explosion risks and impaired working environment.
The production of chlorate requires considerable amounts of hydrochloric acid and alkali metal hydroxide, which means a considerable cost. Besides, the handling of acid and liquor is complicated because of the rigorous safety requirements placed on transport, storage and dosage. Further, the concentration of commercially available products is considerably higher than that immediately usable in the chlorate process.
EP-A-498 484 relates to a process for the production of alkali metal chlorate in combination with chlorine and alkali metal hydroxide, which auxiliary chemicals are used in the chlorate process. In the production of chlorine and alkali metal hydroxide according to this process, an electrolyte containing water from the condenser of the chlorate crystallizer and purified alkali metal chloride is electrolyzed. The production of chlorine and alkali metal hydroxide relates to a chlorine-alkalicell. A catholyte is recirculated in the cathode compartment with a composition of 30 weight-% sodium hydroxide which is equivalent to pH 15. An anolyte of very purified sodium chloride is recirculated in the anode compartment at pH 2 and chlorine gas is produced at the anode. Neither the anolyte nor the catholyte comprises chlorate and to both of them are added very purified water from the evaporation of chlorate electrolyte. The chlorine gas leaves the anode compartment and has to be burned with hydrogen gas to form hydrochloric acid which then can be added to the chlorate process for acidification, or have to be dissolved in a liquid in the process. Although this process entails a reduction of the admission of impurities, it requires extensive equipment, the risk of handling chlorine gas and retains the risk of local, strongly acid regions in the chlorate electrolyte in acidification.